Computer Architecture: Branch Prediction and Hazards

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Questions and Answers

What is a necessary condition for two instructions to be paired together in the decode stage?

They must lack dependencies. (correct)

They must both contain displacements.

They must both be floating point instructions.

They must be dependent on each other.

What happens if two paired instructions are executing concurrently and one stalls due to hazard control?

Both instructions will stall. (correct)

The other instruction will continue executing.

Only the stalled instruction will halt execution.

The processor will discard both instructions and begin again.

What is the primary function of the branch target buffer?

To store immediate values for instructions.

To hold information about all branches. (correct)

To execute the branch instructions.

To manage data dependencies.

In the context of branch prediction, what does 'State 0' indicate?

A branch is very unlikely to jump. Signup and view all the answers

Under what condition will a branch move from State 0 to State 2 in the branch prediction mechanism?

If the branch jumps two times in a row. Signup and view all the answers

What would happen to the pipeline if there is a wrong branch prediction?

It would flush the pipes and wastes cycles. Signup and view all the answers

What is notably incorrect about the Pentium's branch prediction algorithm as described in the text?

State 0 will jump directly to State 3. Signup and view all the answers

What is the initial prediction for a branch that is not found in the branch target buffer?

It will not jump Signup and view all the answers

When does a branch receive its first entry in the branch target buffer?

The first time the branch jumps Signup and view all the answers

What is the primary reason later Pentium models changed their branch prediction mechanism?

To increase the accuracy of branch predictions Signup and view all the answers

Why is a simple two-bit counter insufficient for recognizing some loop patterns?

It cannot store enough previous branch states Signup and view all the answers

What is the role of the 4-bit register in later Pentium branch prediction?

To record the previous behavior of the branch Signup and view all the answers

If the 4-bit register's value is 0001, what does this indicate?

The branch jumped only the last time in 4 attempts Signup and view all the answers

What is the purpose of the 16, 2-bit counters in the improved prediction mechanism?

To predict the next value for specific branch trends Signup and view all the answers

How is the appropriate 2-bit counter selected for a given branch prediction?

Based on the current value of the 4 bit register Signup and view all the answers

What is the main benefit of combining a 4-bit register with 16 updated 2-bit counters?

More accurate branch prediction Signup and view all the answers

What is the primary function of the Next IP unit within the Instruction Fetch Unit (IFU)?

To provide the index for the I-cache. Signup and view all the answers

Which of the following best describes the branch prediction scheme employed in the Pentium II/III microarchitecture?

Two-level adaptive scheme. Signup and view all the answers

What is the primary role of the Branch Target Buffer (BTB) in the Pentium II/III processor?

To maintain branch history and predicted target addresses. Signup and view all the answers

Which unit is responsible for translating complex instructions into simpler micro-operations?

The Microcode Instruction Sequencer. Signup and view all the answers

Where does the instruction fetch unit (IFU) access its instruction data?

The non-blocking I-cache. Signup and view all the answers

What is the primary function of the Return Address Stack?

To predict return addresses for procedure calls. Signup and view all the answers

What is the purpose of P4 branch hints?

To provide information to software to improve trace formation. Signup and view all the answers

How does Out-of-Order Execution enhance performance?

By prioritizing the most common operations in context. Signup and view all the answers

What is the role of the trace cache?

To store micro-ops generated from instructions. Signup and view all the answers

What does 'double-pumped ALU' refer to?

An ALU that executes an operation on each edge of a clock cycle. Signup and view all the answers

What is the function of the retirement stage in the execution pipeline?

To manage and organize completed micro-ops while ensuring the correct order. Signup and view all the answers

What are RAT and ROB used for within register renaming?

To map architectural and physical registers, and to organize completed micro-ops. Signup and view all the answers

How does the L1 Instruction Cache reduce decoder latency?

By storing traces of executed code, directly fetching decodes instructions. Signup and view all the answers

What does it mean for the L1 Data Cache to be non-blocking?

It can handle multiple outstanding load misses, as many as 4. Signup and view all the answers

What kind of replacement algorithm is used for the on-chip caches?

A pseudo-LRU (Least Recently Used) algorithm. Signup and view all the answers

What does a confidence estimator attempt to assess in branch prediction?

The prediction made by the branch predictor. Signup and view all the answers

What characterizes a branch with low confidence?

It changes its direction frequently and irregularly. Signup and view all the answers

Which of the following is NOT a classification category for branch outcomes based on prediction and confidence?

Unpredicted with high confidence U(HC). Signup and view all the answers

How can saturation counter information be used in constructing a confidence estimator?

To speculate more aggressively when the confidence level is higher. Signup and view all the answers

What is a miss distance counter (MDC) table used for in branch prediction?

To determine if a branch has high or low confidence. Signup and view all the answers

In a PAs predictor scheme, when do small numbers of branch history patterns typically lead to correct predictions?

When the patterns match a previously seen pattern. Signup and view all the answers

What is one way that confidence estimation can be used beyond branch prediction?

Thread switching in multithreaded processors. Signup and view all the answers

What is the function of predicate registers in predicated instructions?

To store the conditional execution result. Signup and view all the answers

When does a predicated instruction execute if its predicate is resolved to true?

Only if its predicate is true will execution happen. Signup and view all the answers

What happens to a predicated instruction if its predicate is false?

It is discarded and does not execute. Signup and view all the answers

Study Notes

Intel Pentium Processors

The Intel Pentium processors are a family of microprocessors that have evolved through several generations.
Different generations used varying microarchitectures like P6 and NetBurst.
These processors have advanced functionalities and technologies like MMX, SSE, and Hyper-Threading Technology.
The processors have varying instruction sets and are designed for different applications like general purpose computing and multimedia.

Outline of Topics

Introduction to Intel Architecture
Willamette Processor (11/2000)
Instruction Set Architecture (IA-32)
Instruction Stream
Data Stream
Issues/Problems in Early Pentium Processors
Pentium 4 Revisions: Northwood, Xeon (Prestonia), Prescott
Dual Core Processors and Smithfield
IA-32 (cont'd), Addressing modes and SIMD instruction sets (MMX, SSE)
Pentium Pipelined Integer Unit stages (Pre-fetch, Decode, Address generate, Execution, Writeback)
Superscalar architecture of Pentium Processor
Branch Prediction: the concept and implementation in Pentium Processors
Branch Target Buffer (BTB) and its function
Branch prediction algorithms in later Pentium models
Data types for MMX technology
SIMD execution model
P6 family processors and time frame (1995-1999)
Pentium Pro processor
Dynamic execution and micro-data flow analysis
Cache enhancements
New Generation Chips (Pentium MMX)
Superpipelining and Superscalar
MMX (Multimedia Extensions) features
Pentium III processor and SSE (Streaming SIMD Extensions)
Pentium 4 processor and NetBurst micro-architecture.
Pentium 4 Processor Family (2000-2005)
NetBurst Micro-Architecture, design and components
Streaming SIMD Extensions 2 (SSE2)
Streaming SIMD Extensions 3 (SSE3) and different formats of data types
Hardware support for SSE2
SSE2 instructions (part 1&2)
Pentium 4 instruction stream
Fetching and Decoding Instructions in instruction stream: use of Trace cache
Methods of prefetching
Tracing Cache Function
Branch Prediction in P4 Architecture
Return Address Stack
Out-of-Order Execution
Execution Units (ALU, Load, Store)
In-Order Retirement Section
Pentium II/III Pipeline components, diagrams and details
Pentium II/III summary and offsprings
First-level caches & functionality
Second-level caches and features
400 MHz Intel NetBurst micro-architecture System Bus
Intel Celeron Processor family features, evolution
Overclocking Celeron Processors
Hybrid Branch Predictors
Branch prediction algorithms simulations by Grunwald.
Predicates and Multipath Execution (Confidence Estimation), implementation details
Eager (Multipath) Execution, implementation details
Prediction of Indirect Branches
Branch handling techniques and examples
High-bandwidth branch prediction
Different Versions of Intel Xeon Processors: core features and advancements as servers for high performance computing
Pentium M Processor (2003-2005) and it's purpose for low power computing for mobile devices
Pentium Processor Extreme Edition (2005) and its dual-core technology advances including hyper-threading technology and new advanced features
Pentium III vs. Pentium 4 Pipeline
Comparison between Pentium 3 and Pentium 4 processors (Table format including relevant data points)
Execution benchmarks (using MPEG4, Quake III Arena... on Pentium and related results)
Comparison between Celeron and Duron Processors and their respective results from benchmarks

Key Facts and Entities

IA-32 architecture
MMX (Multimedia Extensions)
SSE (Streaming SIMD Extensions), SSE2, SSE3
Hyper-threading technology
NetBurst micro-architecture
Trace cache
Dynamic execution
Various cache levels
Pentium MMX, Pentium II, Pentium III, Pentium 4, and Intel Xeon processors.
Pentium Pro, Pentium II, Pentium III
Different components of the processors like Execution units, Memory blocks, Caches, pipelines and their functionalities
Different benchmarking techniques

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Description

This quiz delves into the essential concepts of branch prediction and hazard control in computer architecture. It covers various scenarios involving instruction pairing, stalling instructions, and the functionality of the branch target buffer. Test your understanding of how these elements affect pipeline efficiency and performance.